brass monkey weather字面意思把猴子都冻成黄铜了，天气肯定是冷得出奇，所以 brass monkey weather 的意思就是极冷的天气、冷得要命的天气。

【例句】
It's brass monkey weather today. You'd better wrap up warm!
今天真是冷得要命，你最好穿暖和点。

Wow, It's brass monkey weather out there!
哇，外面极冷的天气呢！